Method for assembling a print head

ABSTRACT

A method and jig for assembling a plurality of thermal print wafers to form a print head. The jig includes a fixture plate having accurate first and second planar surfaces perpendicular to each and joined in a common edge. An alignment bar having first and second control surfaces thereon is secured to said plate so that the control surfaces lie in a common plane which is perpendicular to the first planar surface and parallel to the common edge. The print wafers are sandwiched between clamps and positioned on the jig so that the operative ends of the wafers become aligned in a plane controlled by the control surfaces when spring-loaded levers on the jig are used to urge the operative ends against the control surfaces. A compressing lever on the jig is used to compress the wafers between the clamps prior to securing the clamps together to produce a completed print wafer module. The completed print wafer modules are then aligned on a support member while using an alignment fixture having a plurality of control surfaces lying in a common plane. The operative end of each print wafer module is urged into contact with an adjacent pair of control surfaces on the alignment fixture to align the module, and the module is then secured to the support member. The remaining modules are similarly aligned and secured to the support member to produce a completed print head.

METHOD FOR ASSEMBLING A PRINT HEAD v [75] Inventors: Joseph L. Mitchner, Wilmington,

Ohio; Robert A. Keller, Seminole; Clyde B. Graham, St. Petersburg, both of Fla.

[73] Assignee: The National Cash Register Company, Dayton, Ohio [22] Filed: Aug. 6, 1973 [21] Appl. No.: 386,105

Related U.S. Application Data [62] Division of Ser. No. 236,041, March 20, 1973, Pat.

[52] U.S. Cl. 29/466, 29/464 [51] Int. Cl B23q 3/00 [58] Field of Search 29/203 P, 203 J, 464, 466, 29/469; 346/135, 139; 197/1 R; 219/216; 269/228, 239, 287, 315, 319

[56] References Cited UNITED STATES PATENTS 231,604 8/1880 Murry 269/239 1,370,169 3/1921 Turner 269/315 1,885,370 11/1932 Minkow 269/319 2,350,034 5/1944 Herrington.... 269/228 2,639,744 5/1953 Herbert 269/239 Primary ExaminerCharles W. Lanham Assistant Examinerlames R. Duzan Attorney, Agent, or FirmJ. T. Cavender; Albert L.

Sessler, .lr.; Elmer'Wargo 1 1 l I I II I l l l I 1451 May 7, 1974 [57] ABSTRACT A method and jig for assembling a plurality of thermal print wafers to form a print head. The jig includes a fixture plate having accurate first and second planar surfaces perpendicular to each and joined in a common edge. An alignment bar having first and second control surfaces thereon is secured to said plate so that the control surfaces lie in a common plane which is perpendicular to the first planar surface and parallel to the common edge. The print wafers are sandwiched between clamps and positioned on the jig so that the operative ends of the wafers become aligned in a plane controlled by the control surfaces when springloaded levers on the jig are used to urge the operative ends against the control surfaces. A compressing lever on the jig is used to compress the wafers between the clamps prior to securing the clamps together to produce a completed print wafer module. The completed print wafer modules are then aligned on a support member while using an alignment fixture having a plurality of control surfaces lying in a common plane. The operative end of each print wafer module is urged into contact with an adjacent pair of control surfaces on the alignment fixture to align the module, and the module is then secured to the support member. The

- remaining modules are similarly aligned and secured to the support member to produce a completed print head.

2 Claims, 6 Drawing Figures PATENTEDHAY H914 3808.669

SHEET 1. OF 2 minim.

SHEET 2 BF 2 FIG. 4

1 METHOD FOR ASSEMBLING A PRINT HEAD This is a division of application Ser. No. 236,041, filed Mar. 20, 1973, now U.S.-Pat. No. 3,774,899.

This invention was developed under a contract with the United States Government.

BACKGROUND OF THE INVENTION This invention relates to a method and apparatus for assembling a plurality of print wafers to form a print head. I

This invention is especially adaptable for use in forming a thermal print head. A thermal print head generally has a plurality of tiny resistor elements formed on a substrate and arranged in a matrix (like a five by seven matrix) for producing a plurality of character patterns. The resistor elements are generally located in a common plane, and when in use, the resistor elements are brought into contact with a heat sensitive paper. Printing is effected by energizing selected ones of the resistor elements according to the character to be printed. When energized, the resistor elements heat up to a predetermined level causing the heat sensitive .paper in contact therewith to change color, thereby effecting printing of the desired character pattern.

In one type of thermal print head, a plurality of individual resistor elements are formed on an operative end or edge of a print wafer made of a dielectric like a ceramic substrate which is sold under the trademark Fotoceram owned by Owens-Corning Glass Co. The print wafer additionally supports the connecting conductors associated with the elements. Several of these print wafers are stacked one on top of another to form 'a matrix for producing a character pattern. The print wafers are then sandwiched between two clamps which are used to secure the print wafers together so that all their operative ends lie in a common plane thereby producing a print wafer module. Several print wafer modules may be aligned to form aprint head capable of printing a line of characters.

One of the problems with a print head of the type described in the previous paragraph is that unless all the operative ends of print wafers are accurately aligned in a common plane, the quality of the resulting printing produced thereby is poor. Various stacking-andholding fixtures were tried in an effort to align the print head wafers and modules; however, none of them produced the accurate results achieved by the method and apparatus of the present invention.

For military applications of a thermal print head of the type described, it is necessary to provide an apparatus which can be used in the field orat depots for easily assembling the print wafers into modules and for assembling a plurality of modules into a print head for line printing. Completed print heads on printers may be removed therefrom, repaired, and reinstalled in the printers with ease and precision through usingthis invention. I

Some related print heads and print modules are shown in the following US. Pat. Nos.:

3,267,485 which issued. on Aug. 16, 1966, and 3,478,191 which issued on Nov. II, 1969.

SUMMARY OF THE INVENTION This invention relates to a method and apparatus for assembling several print wafers into a module and for assembling several modules into a print head.

The apparatus includes a jig andan alignment fixture. The jig includes a base and a fixture plate secured thereto. The fixture plate has accurate first and second planar surfaces which are perpendicular to each other and are joined along a common edge. An alignment bar having first and second control surfaces thereon is secured to the fixture plate so that the control surfaces lie in a common plane which is perpendicular to the first planar surface and parallel to the common edge. The print wafers are sandwiched between clamps and positioned on the jig so that the operative ends of the wafers become aligned in a plane controlled by the control surfaces when spring loaded first lever means are used to urge the operative ends against the control surfaces. A compressing lever means on the jig is used to compress the wafers between the clamps prior to securing the clamps together to produce a completed print wafer module. The completed print wafer modules are then aligned on a support member while using an alignment fixture having a plurality of control surfaces lying in a common plane. The operative end of each print wafer module is urged into contact with an adjacent pair of control surfaces on the alignment fixture to align the module, and the module is then secured to the support member. The remaining modules are similarly aligned and secured to the support member to produce a completed print head.

' BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a plan view of a jig used in this invention to align a plurality of print wafers against a pair of control 'a plurality of print modules loosely secured to a support member (from a line printer) prior to detachably securing the support member to an .alignment fixture for aligning the print wafer modules.

FIG. 5 is a plan view, in elevation, of the alignment fixture, which view is taken from the direction A of FIG. 4.

FIG. 6 is a cross-sectional view, taken along the line 66 of FIG. 5, showing additional details of control surfaces located on-the alignment fixture and how a print wafer module is aligned thereby.

DETAILED DESCRIPTION OF THE INVENTION FIG. 1 is a plan view of a jig 10 used in this invention to align a plurality of print wafers to form a print wafer module. Because the jig l0 and the method of assembling the print wafers to form a print head are so closely interrelated, both will be discussed together hereinafter.

The jig 10 (FIGS. 1 and 2) includes a base 12 to which a generally rectangular, fixture plate 14, is detachably secured by diagonally positioned locating pins 16 and diagonally positioned fasteners 18. The fixture plate 14 has an accurate planar top surface20'which is positioned perpendicular 'to an accurate planar front surface 22, which surfaces are joined at a common edge. These planar surfaces 20 and 22 are accurate to within 0.0005 inch (flatness). An alignment bar 24, having first and second control surfaces 26 and 28, respectively, thereon is detachably secured to the front planar surface 22 by fasteners 30. The control surfaces 26 and 28 lie in a common plane which is perpendicular to the top surface 20 and is parallel to the common edge joining the surface 20 and 22. In the embodiment shown, the surfaces 26 and 28 are made of 300 Se r ie s stainless corrosion resistant steel. It is against these control surfaces 26 and 28 that the print wafers will be aligned. An alignment pin 32 is positioned near the common edge and the control surface 26 and is secured to the fixture plate 14 so that its axis is perpendicular to the planar surface 20.

The jig also includes a first lever means 34 (FIG. 1) for contacting the rear ends of the print wafers to urge the operative or front ends of the wafers into contact with the first and second control surfaces. The lever means 34 includes a generally L shaped lever 36 which is pivotally mounted on a fastener 38 secured perpendicularly to the fixture plate 14 and another lever 40 which is also pivotally mounted on a fastener 42 which is secured to the fixture plate 14. The lever 36 has a pad area 44 for contacting the rear ends of the print wafers at one side, and similarly, lever 40 has a pad area 46 for contacting the rear ends of the print wafers at the opposite side. Both areas 44 and 46 are planar surfaces which are perpendicular to the top surface 20 and are made of silicon rubber. Both levers 36 and 40 are biased to rotate in a clockwise direction (as viewed in FIG. 1) by associated tension springs 48 and 50. A bar 52 is secured to the fixture plate 14 and is threaded to receive the adjustment screws 54 and 56. When screw 54 is advanced towards lever 36, it rotates lever 36 in a counterclockwise direction to force area 44 thereof into engagement with the rear ends of the print wafers. Similarly, when screw 56 is advanced towards lever 40, it rotates lever 40 in a counterclockwise direction to force area 46 thereof into engagement with the rear ends of the print wafers.

The jig 10 also includes a second lever means 58 (shown in FIGS. 1 and 2) which is used to compress the print wafers while they are positioned on the jig. The lever means 58 includes a frame 60 which is secured to the base 12 as shown. A lever 62 has one end pivotally mounted to the frame 60 by a pin 64 and the remaining end is threaded to receive an adjustable stop 66 with looking nuts 68 thereon. An operating lever 70 is pivotally joined to one end of lever 62 by a pin 72 and it is pivotally joined between its ends to one end of link 74 by a pin 76. The remaining end of link 74 is pivotally joined to the frame 60 by a pin 78. When lever 70 is moved towards the base 12, the lever 62 is rotated in a counterclockwise direction (as' viewed in FIG. 2) to bring the adjustable stop 66 to the position shown where it is used to compress the print wafers as will be later described herein. When the lever 70 is moved towards the base 12, it becomes locked, conventionally, in an above the center position to enable the stop 66 to continually apply a pressure on the print wafers until the lever 70 is moved away from the base 12.

The print wafers shown in the drawings are thermal print wafers. Each wafer, like 80 in FIG. 4, is made of Fotoceram material and has an operative end 82 on which the tiny resistive elements are located. Each wafer has a flexible member 84 which is secured thereto and supports the conducting elements (not' shown) which elements are connected to the resistive elements, and the conducting elements terminate in a isolation diode board 86. Because these aspects of the thermal print wafer may be conventional, they are not shown in detail.

The following method is used to form a print wafer module with the jig 10 shown in FIGS. 1, 2, and 3. The print wafers are sandwiched between upper and lower clamps, so the first step in the method of assembling the module is to detachably secure a lower clamp 88 to the fixture plate by fasteners 90 passing through a hole 92 formed in the base 12 and a portion of the plate 14 as shown in FIG. 2. The next step is to detachably secure the alignment bar 24 to the fixture plate 14. An insulating spacer (like 94 shown in FIG. 4) is then placed on top of the clamp 88, and a first print wafer like 80 is positioned on top of the spacer so that its associated flexible member 84 and connection strip 86 assume the position shown in dashed outline in FIG. 1. This positioning will bring the operative end 82 of the print wafer into contact with the first and second control surfaces 26 and 28. Another spacer like 94 is positioned on top of the first print wafer installed in the jig l0, and a second print wafer is positioned thereover. This process is repeated until five print wafers are installed in the jig (for a five by five print matrix) and the fifth or top print wafer is covered by another insulating spacer, like 96 shown in FIG. 4. An upper clamp 98 is positioned on top of the spacer 96, and fasteners 100 are inserted in the clamps to loosely hold them together. The fasteners 100 pass through notched-out areas 102 (FIG. 3) of the print wafers and insulating spacers. During the time that the print wafers are being installed in the jig 10, the springs 48 and 50 keep their associated levers out of contact with the rear ends of the wafers. When screw 54 (FIG. I) is advanced, area 44 on lever 36 contacts the rear ends 104 of the print wafers to urge the operative ends thereof against the control surface 28. Similarly, when screw 56 is rotated to advance it against the lever 40, the area 46 of lever 40 contacts the rear ends 104 of the print wafers to urge the operative ends thereof against the control surface 26. Lever 40 lies close to the surface 20 and lies under the flexible members 84' associated with the print wafers; however. the lever, at section 106 (FIG. 1) thereof, is angled upwardly away from the surface 20 of fixture plate 14 to enable the associated area 46 to contact the rear ends 104 of the print wafers at the same level as area 44 as shown in FIG. 2. The print wafers are also pushed (in the direction of arrow 108 in FIG. 1) against the pin 32 witha simple square edged wooden pusher to align the wafers thereagainst.

After the screws 54 and 56 (FIG. I) are tightened to force the operative ends of the print wafers 80 into engagement with the-control surfaces 26 and 28, and the wafers are also aligned against the pin 32, the wafers are ready to be clamped between the upper and lower clamps 98 and 88. Prior to clamping, the operating lever 70 is pushed towards the base 12 to bring the adjustable stop 66 into contact with the upper clamp 98 (as shown in FIG. 2) to compress the wafers between the clamps. While so compressed, the screws 100 are tightened to a predetermined torque to hold all the wafers in place. In the embodiment shown, the print wafers, like 80, are generally rectangular in shape having a length of 1.6 inches, a width of 1.0 inches, and a thickness of 0.020 inch. With five such wafers forming a module, the torque applied to the screws 100 was 601:2 inch ounces. Naturally, the dimensions given are merely representaive of print wafers in general.

After the screws 100 (FIG. 1) have been tightened to a predetermined torque, the lever 62 which compresses the print .wafers is removed, and the resulting print wafer module is inspected. To inspect the module, the alignment bar 24 is removed permitting visual inspection of the operative ends of the wafers. The print wafer module, while still attached to the fixture plate, is then placed on an accurate flat surface so that the front surface 22 of the fixture plate 14 and the operative ends of the print wafer module are perpendicular to an accurateflat surface. If all operative edges of the print wafers do not contact the accurate flat surface to within 0.0003 inch, the print module is defective and the module must be reassembled after removing any defective print wafers. An air gauge may also be conventionally used to measure whether or not an individual wafer contacts the flat accurate surface. Those print wafer modules which pass inspection can then be removed from the jig by removing fasteners 90.

The completed print wafer modules are assembled to form a print head by the method shown in FIGS. 4, 5, and 6. An alignment fixture 110 is used to align the completed print wafer modules like 112 (FIG. 4) on a support member 114 which is part of a printer (not shown) in which the modules are used.

The alignment fixture 110 has a base 116 which may be detachably secured to a stable workbench when in use. The fixture also includes a generally U-shaped member having the legs 118 and 120 thereof secured to the base 116 by fasteners 122.The fixture 110 has an accurate planar surface 124 which is parallel to the faces 126 and 128 of legs 118 and 120, respectively, and perpendicular to the top surface of base 116 as viewed in FIG. 4 A plurality of holes like 130 is drilled into the planar surface 124 so that the axes of the holes are perpendicular to the surface 124, are located on equal centers, and are all located along a straight line which is parallel to the top surface of the base 116. The holes 130 are spaced apart on centers a distance which is slightly longer than the length of the modules 112 being aligned thereby. A locating pin 132 is positioned in each of the holes 130 so that a portion of the pin projects from the surface 124. Each of the pins 132 has a control, surface which is perpendicular to the axis of its associated hole, and all the control surfaces of the pins 132 lie in a common plane which is parallel to the surface 124.

To align the print wafer modules 112 on the support member 114, the following procedure is used. The print wafer modules 112 are loosely secured to the top surface 134 of the support member 114 by suitable fasteners 136 as shown in FIG. 4; The top surface 134 is an accurate planar surface which is perpendicular to the flat mounting surfaces 136 and 138 of the member 114. The support member 114 is then detachably secured to the alignment fixture 110 by bringing the mounting surfaces 136 and 138 thereof into aligned engagement with the faces 126 and 128 of the fixture 110 through using suitable locating pins 140 and fasteners 142 as shown in FIG. 4. The center print wafer module 144 (identical to modules 112) is manually pushed against the control surfaces of two adjacent locating pins 132 so that the operative ends of the print wafers of the module contact the control surfaces of the pins 132. While so contacting the control surfaces, the associated screws 136 are tightened to secure the lower clamp 88 of the print module to the surface 134 of the support member 114. In the embodiment described, the screws 136 were tightened to a torque of 8 to 10 inch pounds. The print wafer modules 112 on each side of the center module 144 are similarly aligned and fastened to the support member 114; however, a thin insulating spacer 146 (FIG. 4) is placed between adjacent print wafer modules prior to being fastened to the support member 114. Notice in FIG. 5 that adjacent print wafer modules share a common control surface of a pin 132 during the alignment procedure. After all the print wafer modules are secured to the support member 114, member 114 is detached from the alignment fixture and it is ready to be installed in a printer. The print wafer modules, after alignment, form a print head for producing a line of printing. Various warm up heater plate elements like 148 and 150 may be installed under and over the associated print modules as shown in FIG. 1 without affecting the alignment procedure. The fixture 110 is made of anodized aluminum tool plate, and the pins 132 were made of brass.

What is claimed is:

' 1. A method of assemblying a thermal print head composed of a plurality of print wafer modules, comprising the steps of:

a. aligning a plurality of print wafers, each having an operative end and a rear end, so that the print wafers are superimposed on one another in aligned relationship and all the operative ends thereof lie in a common plane;

b. compressing theplurality of print wafers between upper and lower clamp members while aligned as in step (a); g

c. securing said upper and lower'clamps together to sandwich said print wafers therebetween to hold said operative'ends in said common plane and thereby produce one of said print wafer modules;

(I. loosely attaching a plurality of said print wafer modules on a support member so that all the operative ends of the print wafers of the modules lie substantially in a common plane;

e. detachably. securing said support member to an alignment fixture having a plurality of aligned, spaced, control surfaces lying in a common plane;

said one secured at the center of the support member,

and thereafter securing the remaining modules in a similar manner while simultaneously working from the center of the support member towards the ends thereof. 

1. A method of assemblying a thermal print head composed of a plurality of print wafer modules, comprising the steps of: a. aligning a plurality of print wafers, each having an operative end and a rear end, so that the print wafers are superimposed on one another in aligned relationship and all the operative ends thereof lie in a common plane; b. compressing the plurality of print wafers between upper and lower clamp members while aligned as in step (a); c. securing said upper and lower clamps together to sandwich said print wafers therebetween to hold said operative ends in said common plane and thereby produce one of said print wafer modules; d. loosely attaching a plurality of said print wafer modules on a support member so that all the operative ends of the print wafers of the modules lie substantially in a common plane; e. detachably securing said support member to an alignment fixture having a plurality of aligned, spaced, control surfaces lying in a common plane; f. accurately aligning each of said modules by urging the operative end thereof into engagement with two adjacent ones of said control surfaces; and g. securing each aligned module from step (f) to said support member.
 2. The method as claimed in claim 1 in which said securing step (g) is accomplished by securing one of said modules at the center of said support member, and thereafter securing the remaining modules to said support member by securing the modules next adjacent to said one secured at the center of the support member, and thereafter securing the remaining modules in a similar manner while simultaneously working from the center of the support member towards the ends thereof. 